1. Field of the Invention
The present invention relates to a rolling bearing and, in particular, to an improvement in the life of a rolling bearing used, for example, in transmissions and engines for automobiles, agricultural machines, construction machines and iron and steel machines.
2. Description of the Prior Art
Foreign substances entrained in a bearing lubricant are one of the factors which reduce the life of a rolling bearing. It has been known that cutting powders, scraping dusts, burrs and abrasion powders of metals are entrained in the bearing lubricant and injure the bearing rings and/or the rolling members of the rolling bearing so as to reduce the life of the rolling bearing. Then, there has been a problem that the bearing life is reduced to about 1/10 as compared with the case of a bearing employing a lubricant with no entrained foreign substances.
Further, as in the case of pitching caused in transmission gears in automobiles, spot-like minute pores are formed on the bearing surface of the rolling surface of the bearing due to rolling fatigue which reduce the life of the rolling bearing. In view of the above, it has been known, such as disclosed in Japanese Patent Publication Sho 62-24499 and Japanese Patent Laid-Open Hei 2-34776, to provide a low-medium carbon low alloy steel with improved hardness of the steel surface and improved pitching resistance in which spherical carbides are precipitated to the surface of a low-to-medium carbon or a low-alloy steel by a heat treatment, such as carburization.
When the surface hardness of the bearing rings and the rolling members is improved as in the prior art described above, any impressions due to the foreign substances are moderated. On the contrary, toughness of the bearing ring and the rolling member becomes poor and a crack is formed from a portion injured by the foreign substances existing in the lubricant. This acts to initiate flaking from such portions at an early stage and, accordingly, there is a limit for the improvement of the life of the rolling bearing.
In view of the above, U.S. Pat. No. 4,904,094 was proposed by several of the present Applicants in which stress concentration at the edge portion of the impression caused by foreign substances is relaxed to suppress the formation of a crack and to thereby improve the life of the rolling bearing by adjusting the content of carbon, the content of retained austenite and the content of carbonitride in the rolling surface layer of the rolling bearing to appropriate values even in a case where the rolling bearing is used under lubrication with entrained foreign substances.
In the prior art disclosed in U.S. Pat. No. 4,904,094, the life of a rolling bearing under lubrication with entrained foreign substances can be improved by an appropriate content of retained austenite; but, on the other hand, there is a problem that the surface hardness is lowered by the retained austenite which reduces the fatigue resistance. That is, there is still room for the improvement of an appropriate relationship between the content of the retained austenite and the surface hardness.
Further, in the prior art bearing material, no consideration is taken of determining the value for the grain size of carbides and carbonitrides for improving the life of the bearing. When large sized carbides undergo repeated stresses, there is a problem that the large-sized carbides act as initiation sites for fatigue to cause cracks and flaking.
In view of the above, an object of the present invention for eliminating such a problem is to provide a rolling bearing having a longer life than that of the prior art products even under clean lubrication as well as under lubrication with entrained foreign substances by finding an optimum relationship between the content of the retained austenite and the surface hardness in a rolling surface layer and, further, by adjusting the average grain size of carbides and carbonitrides existing in the rolling surface layer to optimum values.